Toronto police will be allowed to use sound cannons with some restrictions to disperse G20 protesters, a judge has ruled.

Civil liberties advocates and activists had requested that a court impose an injunction to prevent police from using a device known as Long Range Acoustic Devices.

Toronto police will be able to use the LRAD's voice function, but may only use a function which emits an ear-piercing pulse once every thirty seconds, Justice David Brown of the Ontario Superior Court ruled Friday.

"It's an important aspect of keeping people safe in our city, including protesters. We believe it is safe," police chief Bill Blair said Friday.

Earlier Ontario Federation of Labour president Sid Ryan expressed concern that police have gone too far with G20 security and will intimidate busloads of people who are coming to Toronto for a peaceful march on Saturday.

"We're trying to put on here a family-friendly protest," Ryan told reporters before an application was submitted to the court.

"We want families to come out. We want them to bring their children out. We want them to bring their relatives out. No one should be afraid of marching with the labour movement, and yet we've got this armed force that's in our city, the likes of which I've never seen."